Effective particle deposition processes, such as thermal spray processes, are limited by a range of spray process variables that are necessary to produce coatings having limited ranges of acceptable microstructures and morphologies. The operating parameters of a thermal spray process include these spray process variables as well as fixed process parameters. Spray process operating parameters result in a window of coating microstructure and morphology that is a direct result of complex interactions of process and material property variables during the spray process. These complex interactions combine to form two measurable in-flight particle characteristics or properties that contribute to coating microstructure and morphology. These are the temperature and velocity distributions of in-flight particles. Standard or classic thermal spray applications typically produce Gaussian or normal in-flight particle temperature and velocity distributions that may be measured using known instruments, such as the DPV 2000 manufactured by TECNAR. The result of using Gaussian distributions is typically a single output of average particle temperature and velocity with a defined deviation for a given set of spray process operating parameters and/or coating specifications. This results in only a small window of coating microstructure and morphology for that specific spray process.
Changing the average value of the Gaussian distributions for in-flight particle temperature and velocity makes it possible to change the average characteristics of the coating being deposited. The average characteristics are those microstructurally and morphologically important properties of a coating that affect its performance. In general, as the temperature and velocity of in-flight particles are increased, the particles become more molten and impact at a higher rate forming a denser coating. As the temperature and velocity are increased even further, splashing begins to occur as well as foreign matter entrapment. As the temperature and velocity are lowered, porosity becomes higher and the presence of un-melted particles increases causing a weaker coating to form. Changing the standard deviation without changing the average values of the temperature and velocity distributions inherently affects the non-uniformity of the deposited coating. This is because there is a broader range of temperature and velocity over which the particles are spread during deposition. In general, the standard or classic approach in a thermal spray process has been to reduce the standard deviations around a central or narrow range of temperature and velocity average distribution values that is known to deposit an acceptable coating.
U.S. Pat. No. 5,817,372 discloses a method of depositing a bond coat of a thermal barrier coating system by choosing two particle powders having different sizes. The particle size distributions of the two powders are chosen to yield a bond coat characterized by a particular macro-surface roughness attributable to particles of a coarser powder.